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PHENOLS
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PHENOL

Phenol are the aromatic compounds having hydroxyl group (-OH) directly attached to benzene ring.
ELECTRONIC STRUCTURE OF PHENOL

In phenols the –OH group is attached to sp2 hybrid carbon of an aromatic ring and the oxygen atom of
the hydroxyl group has two lone pairs of electrons and the bond angle in phenol is 109o. The C-O bond
length in phenol (136pm) is slightly less than in methanol(412pm) due to resonance in aromatic ring of
phenol

Phenol has dipole moment 1.54D where as methanol has dipole moment 1.71D. This smaller dipole
moment of phenol is due to the electron attracting effect of phenyl group in contrast to the electron
releasing effect of methyl ( or alkyl) group in alcohol
CLASSIFICATION OF PHENOL

Phenols are classified as mono, di and trihydric phenols according to the number of hydroxyl groups
attached to the aromatic ring.
1.
Monohydric phenols
2.
Dihydric phenols
3.
Trihydric phenols
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GENERAL METHODS OF PREPARATION OF PHENOLS
1.
Alkali fusion of sulphonates
Sodium salt of aryl sulphonic acids on fusion with sodium hydroxide at 300-350oC yield phenol
The above reaction is laboratory method for preparation of phenol
2.
Hydrolysis of diazonium salt
3.
Hydrolysis of aryl halides ( Dow’s process)

Aryl halides on hydrolysis yield phenol but the process is not so simple because halogen atom attached
to benzene ring does not undergo SN2 reaction since C-X bond is resonance stabilized. Hence reaction
takes place at high pressure and elevated pressure.
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
However if some electron withdrawing group is attached to benzene ring, then reaction conditions get
relaxed
4.
Decarboxylation of salicylic acid
5.
Oxidation of Grignard reagent
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6.
Oxidation of aromatic hydrocarbons
Catalyst used in this reaction is cupric salt.
7.
Oxidation of iso-propyl benzene (Cumene)
Cumene is prepared by Friedel Crafts alkylation of benzene with 2-Chloropropane.
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8.
Rasching’s process
PHYSICAL PROPERTIES OF PHENOL
1.
Physical state

Phenols are colourless liquids or low melting point solids, but they reddish brown due to auto oxidation
on exposure to air and light.

Phenols are poisonous in nature but act as disinfectant and antiseptic.
2.
Solubility
Phenols form H-Bonds with water molecule and hence soluble in water, but their solubility is lower than
that of alcohols because of large hydrocarbon part
Intermolecular H-bonding among water and phenol molecules
3.
Boiling point

Phenols have much higher boiling point than there corresponding hydrocarbons due to intermolecular
hydrogen bonding

Amongst the isomeric nitrophenols, O-nitrophenol has much lower melting point and solubility than
meta and para isomers of nitrophenol because of intramolecular H-bonding. Thus it does not undergo
association with other molecules. In fact ortho-nitrophenol is steam volatile

On the other hand meta and para nitrophenol exhibit intermolecular H-bonding with their own
molecules as well as with water molecules and hence show comparatively higher melting and solubility.
i)
Intermolecular H-bonding ( m –nitrophenol)
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ii)
H- bonding with water molecules ( m-nitrophenol)
iii)
Intermolecular H- bonding ( p –nitrophenol)
iv)
H – bonding with water molecules ( p – nitrophenol)
CHEMCIAL PROPERTIES OF PHENOL
(A) Reactions involving cleavage of O-H bond
1.
Acidic character of phenol
(i)
Reaction with active metals
(ii)
Reaction with alkalies
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
They do not react with carbonates and bicarbonates. Phenols are weaker acid as compared to
carboxylic acid because of polar O-H group in them. The acidic nature of phenol is due to formation of
stable phenoxide ion in solution to give H+ ions

Phenol behaves as stronger acid than alcohol
i.
The greater acidity of phenol is due to stability of phenoxide ion is resonance stabilised .
ii.
Resonance structure of phenol
iii.
As a result of resonance it weakens the polar O-H bond and thus facilitates the release of
proton (H+) to give phenoxide ion which is also stable due to resonance
iv.
Resonating structure of phenoxide ion
v.
Both phenol and phenoxide ion are stable due to resonance. But phenoxide ion is more
stabilized than phenol because the resonating structure of phenoxide ion carry only negative
structure of phenol involves separation of positive and negative charge. This charge
delocalization is a stabilizing factor in phenoxide ion and increases acidity of phenol.
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vi.
a.
Effect of substituent on the acidity of phenols
Electron withdrawing group (EWG)
1.
Electron withdrawing groups NO2 , -X, -CHO, -COOH, -CN etc stabilizes the phenoxide ion more
by dispersing the negative charge relative to phenol ( i.e. proton release become easy) and
thus increases the acidic strength of phenols
2.
Particular effect is more when the substituent is present on O – and p- positions than in mposition. Thus acidic strength of nitrophenol decreases in the order
p-nitrophenol > o-nitrophenol > m-nitrophenol > phenol
3.
Further greater the number of electron withdrawing groups at o- and
p – position, more
acidic the phenol.
b.
Electron donating group (EDG)
(i)
Electron donating group –R, -NH2, -OR etc destabilize the phenoxide ion by donating
electrons and intensify the negative charge relative to phenol (i.e. proton release become
difficult)and thus decreases the acidic strength of phenol.
(ii)
The effect is more when the substituent is present on o- and p- position than on mposition with respect to –OH group. Thus cresol are less acidic than phenol.
m – methoxy and m-amnion phenols are stronger acid than phenols because of –I effect ( of –
OCH3 and –NH2 groups) and absence of +R effect.
m –methoxy phenol > m-amino phenol > phenol > O-methoxy phenol > p-methoxy phenol
vii)
Reaction of –OH group
i.
Reaction with FeCl3 is the test of phenol
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3C6H5OH + FeCl3  (C6H5O)3Fe + 3HCl
(C6H5O)3Fe : Ferric phenoxide ( violet complex)
ii.
Ester formation
Reaction with benzoyl chloride is known as benzoylation
Benzoylation of alcohols o phenols in presence of NaOH is called Schotten-Baumann reaction.
iii.

Ether formation
Claisen rearrangement
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(B) Electrophilic aromatic substitution reaction
1.
Halogenation
Due to steric hinderance at ortho position, para-product predominates
2.
Sulphonation
3.
Nitration
4.
Mercuration
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5.
Friedel crafts alkylation and acylation
6.
Nitrosation
7.
Kolbe’s reaction
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Mechanism
(a) Acetyl salicylic acid ( Asprine)
It is a white solid (m.pt. 408K) and is used for relieving pain (analgesic) and to bring down the
body temperature (antipyretic during fever)
(b) Methyl salicylate
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It is an oily liquid (b.pt. 495K) with pleasant odour (oil of wintergreen) used in perfumery and
flavouring agent.
It is also used in medicine in the treatment of rheumatic pain and as a remedy for aches,
sprains and bruises.
(c) phenyl salicylate ( salol)
It is white solid (m.pt 316K) and is used as an intestinal antispeptic.
8.
Reimer-Tiemann reaction
Mechanism
The electrophile, dichloromethylene CCl2 is generated from chloroform by action of base
OH- + CHCl3 ⇆ HOH + CCl3 →Cl- + CCl2
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Attack of electrophile ( :CCl2) on phenoxide ion
Salicyaldehyde
If the reaction is carried out with carbon tetrachloride (CCl4) instead of chloroform, o-hydroxy benzoic
acid (salicylic acid) is formed.
TESTS TO DISTINGUISH BETWEEN ALCOHOLS AND PHENOLS
1.
Litmus test
Phenol turns blue litmus red being acidic in nature but alcohol do not.
2.
Ferric chloride test
Phenol react with neutral ferric chloride solution to form violet or green coloured solution where as
alcohol do not undergo such reactions.
3.
Bromine water test
Aqueous solution of phenol forms white precipitate of 2,4,6-tribromo phenol, when treated with
bromine water. However alcohol so not give such precipitation.
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